A wireless local-area networks (WLAN) links devices using a wireless distribution method (such as orthogonal frequency-division multiplexing (OFDM)). A WLAN allows users to move around an area serviced by the WLAN while still maintaining interconnectivity. For this reason WLANS have become increasingly popular with consumers both in the home and in commercial and public areas.
Improvements in communication technology drive the increase of wireless data rate in wireless local area networks (WLANs). For example, the latest ratified Institute of Electrical and Electronics Engineers (IEEE) 802.11n standard has boosted the physical layer (PHY) data rate to hundreds of megabytes per second (Mbps). The physical layer is the first and lowest layer in the seven-layer Open System Interconnection (OSI) model of computer networking.
The increase in the data rate in the PHY is mainly due to wider channel bandwidth and advanced modulation techniques like Multiple-Input Multiple-Output (MIMO). Future standards like the IEEE 802.11 ac standard and the 802.11ad standard are already poised to provide even faster PHY speeds on the order of gigabytes per second (Gbps).
However, the throughput efficiency, which is the ratio between the network throughput and the PHY data rate, has degraded rapidly as the PHY data rate has increased. One reason for this is the inadequacy of current IEEE 802.11 medium access control (MAC) data communication protocol. The MAC is a sublayer of the data link layer, which is the second layer of the OSI networking model. For example, given that most internet protocol (IP) packets have a maximal transmit unit (MTU) size around of 1500 bytes, the efficiency ratio in an 802.11n network at 300 Mbps is around twenty percent. In other words, the 300 Mbps data rate can sustain an actual throughput of only about 60 Mbps.
One reason for such inefficiency is that the current MAC data communication protocol allocates the entire channel as a single unit. Such allocation can become too coarse grained when the channel width increases or PHY data rate increases. Even if a sender has a small amount of data to send the data still has to compete for the whole channel. The time spent resolving this contention for the channel therefore becomes an overhead to the useful channel time. Unfortunately, this overhead cannot be easily reduced due to physical and electronics constraints. As a result, the higher the PHY data rate the lower the throughput efficiency.
Attempts have been made to deal with this lack of WLAN efficiency. One way to improve efficiency is to extend the useful channel time on data transmission by sending larger frames. Indeed, the IEEE 802.11n standard allows frame aggregation (or sending multiple frames together). However, when the PHY data rate goes up the aggregated frame size needs to go up as well. This means to achieve an efficiency of eighty percent in a 300 Mbps network it would require the frames to be as big as 23 KB. This means longer delays as the sender waits to collect enough frames before actual transmission, which adversely affects transport control protocol (TCP), real-time applications like voice over IP (VoIP), video conferencing, and to web browsing. Indeed, tests have shown that a network throughput of 300 Mbps an 802.11n network can only increase to around 80 Mbps after frame aggregation is enabled.
One way to improve WLAN efficiency is by reducing the channel width and creating additional channels. Each node in the network may transmit on these small channels simultaneously, thereby amortizing MAC coordination overhead among multiple users. One problem, however, with this idea is that it is difficult to slice a channel band into multiple subchannels without losing useful bandwidth.
One common practice in creating subchannels is to waste both edges of two adjacent subchannels as “guard band” so that the useful transmissions are properly spaced out to avoid interfering with each other. However, these guard bands can add up to significant overhead, especially as the number of subchannels increase. Moreover, no matter the width of a subchannel the guard-band width cannot be easily reduced due to the power mask requirement.
In order to avoid the need for guard bands, some WLANs use the orthogonal frequency division multiplexing (OFDM) wireless distribution method. OFDM is a well understood PHY-layer technology that works best if the frequency and width of subchannels are strategically picked and transmission on each subchannel is synchronized in a way to become “orthogonal” (and thus non-interfering) to one another. Although some cellular networks have proposed to use OFDM in channel multi-access (OFDMA), it requires tight synchronization among user handsets and it does not support random access. It thus remains a technical challenge on how to use OFDM-type channelization for dividing a channel into subchannels among distributed and asynchronous stations in a random-access WLAN, where it is impractical and unnecessary to achieve the same tight synchronization.